The fission yeast kinetochore component Spc7 associates with the EB1 family member Mal3 and is required for kinetochore-spindle association

Aurora B kinase erases monopolar microtubule-kinetochore arrays at the meiosis I-II transition

During meiosis, faithful chromosome segregation requires monopolar spindle microtubule-kinetochore arrays in MI to segregate homologous chromosomes, but bipolar in MII to segregate sister chromatids. Using fission yeasts, we found that the universal Aurora B kinase localizes to kinetochores in metaphase I and in the mid-spindle during anaphase I, as in mitosis; but in the absence of an intervening S phase, the importin α Imp1 propitiates its release from the spindle midzone to re-localize at kinetochores during meiotic interkinesis. We show that "error-correction" activity of kinetochore re-localized Aurora B becomes essential to erase monopolar arrangements from anaphase I, a prerequisite to satisfy the spindle assembly checkpoint (SAC) and to generate proper bipolar arrays at the onset of MII. This microtubule-kinetochore resetting activity of Aurora B at the MI-MII transition is required to prevent chromosome missegregation in meiosis II, a type of error often associated with birth defects and infertility in humans.

Genome-Wide Analysis of the MADS-Box Gene Family in Maize: Gene Structure, Evolution, and Relationships

The MADS-box gene family is one of the largest families in plants and plays an important roles in floral development. The MADS-box family includes the SRF-like domain and K-box domain. It is considered that the MADS-box gene family encodes a DNA-binding domain that is generally related to transcription factors, and plays important roles in regulating floral development. Our study identified 211 MADS-box protein sequences in the Zea mays proteome and renamed all the genes based on the gene annotations. All the 211 MADS-box protein sequences were coded by 98 expressed genes. Phylogenetic analysis of the MADS-box genes showed that all the family members were categorized into five subfamilies: MIKC-type, Mα, Mβ, Mγ, and Mδ. Gene duplications are regarded as products of several types of errors during the period of DNA replication and reconstruction; in our study all the 98 MADS-box genes contained 22 pairs of segmentally duplicated events which were distributed on 10 chromosomes. We compared expression data in different tissues from the female spikelet, silk, pericarp aleurone, ear primordium, leaf zone, vegetative meristem, internode, endosperm crown, mature pollen, embryo, root cortex, secondary root, germination kernels, primary root, root elongation zone, and root meristem. According to analysis of gene ontology pathways, we found a total of 41 pathways in which MADS-box genes in maize are involved. All the studies we conducted provided an overview of MADS-box gene family members in maize and showed multiple functions as transcription factors. The related research of MADS-box domains has provided the theoretical basis of MADS-box domains for agricultural applications.

Tell the Difference Between Mitosis and Meiosis: Interplay Between Chromosomes, Cytoskeleton, and Cell Cycle Regulation

Meiosis is a specialized style of cell division conserved in eukaryotes, particularly designed for the production of gametes. A huge number of studies to date have demonstrated how chromosomes behave and how meiotic events are controlled. Yeast substantially contributed to the understanding of the molecular mechanisms of meiosis in the past decades. Recently, evidence began to accumulate to draw a perspective landscape showing that chromosomes and microtubules are mutually influenced: microtubules regulate chromosomes, whereas chromosomes also regulate microtubule behaviors. Here we focus on lessons from recent advancement in genetical and cytological studies of the fission yeast Schizosaccharomyces pombe, revealing how chromosomes, cytoskeleton, and cell cycle progression are organized and particularly how these are differentiated in mitosis and meiosis. These studies illuminate that meiosis is strategically designed to fulfill two missions: faithful segregation of genetic materials and production of genetic diversity in descendants through elaboration by meiosis-specific factors in collaboration with general factors.

Glucose limitation and pka1 deletion rescue aberrant mitotic spindle formation induced by Mal3 overexpression in Schizosaccharomyces pombe

The cAMP-dependent protein kinase Pka1 is known as a regulator of glycogenesis, transition into meiosis, proper chromosome segregation, and stress responses in Schizosaccharomyces pombe. We demonstrated that both the cAMP/PKA pathway and glucose limitation play roles in appropriate spindle formation. Overexpression of Mal3 (1-308), an EB1 family protein, caused growth defects, increased 4C DNA content, and induced monopolar spindle formation. Overproduction of a high-affinity microtubule binding mutant (Q89R) and a recombinant protein possessing the CH and EB1 domains (1-241) both resulted in more severe phenotypes than Mal3 (1-308). Loss of functional Pka1 and glucose limitation rescued the phenotypes of Mal3-overexpressing cells, whereas deletion of Tor1 or Ssp2 did not. Growth defects and monopolar spindle formation in a kinesin-5 mutant, cut7-446, was partially rescued by pka1 deletion or glucose limitation. These findings suggest that Pka1 and glucose limitation regulate proper spindle formation in Mal3-overexpressing cells and the cut7-446 mutant.

The novel microtubule-associated CAP-glycine protein Cgp1 governs growth, differentiation, and virulence of Cryptococcus neoformans

Microtubules are involved in mechanical support, cytoplasmic organization, and several cellular processes by interacting with diverse microtubule-associated proteins such as plus-end tracking proteins, motor proteins, and tubulin-folding cofactors. A number of the cytoskeleton-associated proteins (CAPs) contain the CAP-glycine-rich (CAP-Gly) domain, which is evolutionarily conserved and generally considered to bind to α-tubulin to regulate the function of microtubules. However, there has been a dearth of research on CAP-Gly proteins in fungal pathogens, including Cryptococcus neoformans, which is a global cause of fatal meningoencephalitis in immunocompromised patients. In this study, we identified five CAP-Gly protein-encoding genes in C. neoformans. Among these, Cgp1 encoded by CNAG_06352 has a unique domain structure containing CAP-Gly, SPEC, and Spc7 domains that is not orthologous to CAPs in other eukaryotes. Supporting the role of Cgp1 in microtubule-related function, we demonstrate that deletion or overexpression of CGP1 alters cellular susceptibility to thiabendazole, a microtubule destabilizer and that Cgp1 is co-localized with cytoplasmic microtubules. Related to the cellular function of microtubules, Cgp1 governs the maintenance of membrane stability and genotoxic stress responses. Deletion of CGP1 also reduces production of melanin pigment and attenuates the virulence of C. neoformans. Furthermore, we demonstrate that Cgp1 uniquely regulates the sexual differentiation of C. neoformans with distinct roles in the early and late stage of mating. Domain analysis revealed that the CAP-Gly domain plays a major role in all Cgp1 functions examined. In conclusion, this novel CAP-Gly protein, Cgp1, has pleotropic roles in regulating growth, stress responses, differentiation, and virulence in C. neoformans.

Investigating the Molecular Genetic Basis of Cytoplasmic Sex Determination Caused by Wolbachia Endosymbionts in Terrestrial Isopods

In animals, sexual differences between males and females are usually determined by sex chromosomes. Alternatively, sex may also be determined by vertically transmitted intracellular microbial endosymbionts. The best known cytoplasmic sex manipulative endosymbiont is Wolbachia which can, for instance, feminize genetic males into phenotypic females in the terrestrial isopod Armadillidium vulgare. However, the molecular genetic basis of cytoplasmic sex determination is unknown. To identify candidate genes of feminization induced by Wolbachia strain wVulC from A. vulgare, we sequenced the genome of Wolbachia strain wCon from Cylisticus convexus, the most closely related known Wolbachia strain to wVulC that does not induce feminization, and compared it to the wVulC genome. Then, we performed gene expression profiling of the 216 resulting wVulC candidate genes throughout host developmental stages in A. vulgare and the heterologous host C. convexus. We identified a set of 35 feminization candidate genes showing differential expression during host sexual development. Interestingly, 27 of the 35 genes are present in the f element, which is a piece of a feminizing Wolbachia genome horizontally transferred into the nuclear genome of A. vulgare and involved in female sex determination. Assuming that the molecular genetic basis of feminization by Wolbachia and the f element is the same, the 27 genes are candidates for acting as master sex determination genes in A. vulgare females carrying the f element.

An unconventional interaction between Dis1/TOG and Mal3/EB1 in fission yeast promotes the fidelity of chromosome segregation

Dynamic microtubule plus-ends interact with various intracellular target regions such as the cell cortex and the kinetochore. Two conserved families of microtubule plus-end-tracking proteins, the XMAP215, ch-TOG or CKAP5 family and the end-binding 1 (EB1, also known as MAPRE1) family, play pivotal roles in regulating microtubule dynamics. Here, we study the functional interplay between fission yeast Dis1, a member of the XMAP215/TOG family, and Mal3, an EB1 protein. Using an in vitro microscopy assay, we find that purified Dis1 autonomously tracks growing microtubule ends and is a bona fide microtubule polymerase. Mal3 recruits additional Dis1 to microtubule ends, explaining the synergistic enhancement of microtubule dynamicity by these proteins. A non-canonical binding motif in Dis1 mediates the interaction with Mal3. X-ray crystallography shows that this new motif interacts in an unconventional configuration with the conserved hydrophobic cavity formed within the Mal3 C-terminal region that typically interacts with the canonical SXIP motif. Selectively perturbing the Mal3-Dis1 interaction in living cells demonstrates that it is important for accurate chromosome segregation. Whereas, in some metazoans, the interaction between EB1 and the XMAP215/TOG family members requires an additional binding partner, fission yeast relies on a direct interaction, indicating evolutionary plasticity of this critical interaction module.

A chaperone-assisted degradation pathway targets kinetochore proteins to ensure genome stability

Cells are regularly exposed to stress conditions that may lead to protein misfolding. To cope with this challenge, molecular chaperones selectively target structurally perturbed proteins for degradation via the ubiquitin-proteasome pathway. In mammals the co-chaperone BAG-1 plays an important role in this system. BAG-1 has two orthologues, Bag101 and Bag102, in the fission yeast Schizosaccharomyces pombe. We show that both Bag101 and Bag102 interact with 26S proteasomes and Hsp70. By epistasis mapping we identify a mutant in the conserved kinetochore component Spc7 (Spc105/Blinkin) as a target for a quality control system that also involves, Hsp70, Bag102, the 26S proteasome, Ubc4 and the ubiquitin-ligases Ubr11 and San1. Accordingly, chromosome missegregation of spc7 mutant strains is alleviated by mutation of components in this pathway. In addition, we isolated a dominant negative version of the deubiquitylating enzyme, Ubp3, as a suppressor of the spc7-23 phenotype, suggesting that the proteasome-associated Ubp3 is required for this degradation system. Finally, our data suggest that the identified pathway is also involved in quality control of other kinetochore components and therefore likely to be a common degradation mechanism to ensure nuclear protein homeostasis and genome integrity.

The accumulation of misfolded proteins represents a considerable threat to the health of individual cells and has been linked to severe diseases, including cancer and neurodegenerative disorders. To cope with this threat, especially under stress conditions, cells have evolved efficient quality control mechanisms. In general, these rely on molecular chaperones to either seize and refold misfolded proteins, or target them for degradation via the ubiquitin-proteasome system. At present, our understanding of what determines whether a chaperone commits to a folding or a degradation mode is limited. However, studies suggest that association with certain regulatory co-chaperones contributes to this process. Here, we show that certain BAG-1-type co-chaperones function in quality control by targeting misfolded kinetochore components for proteolysis. The presented genetic and biochemical data show that specific ubiquitin conjugating enzymes and ubiquitin-protein ligases maintain nuclear protein homeostasis and are required for upholding genome integrity.

KNL1: bringing order to the kinetochore

KNL1 is an evolutionarily conserved kinetochore-associated protein essential for accurate chromosome segregation in eukaryotic cells. This large scaffold protein, predicted to be almost entirely unstructured, is involved in diverse mitotic processes including kinetochore assembly, chromosome congression, and mitotic checkpoint signaling. How this kinetochore "hub" coordinates protein-protein interactions spatially and temporally during mitosis to orchestrate these processes is an area of active investigation. Here we summarize the current understanding of KNL1 and discuss possible mechanisms by which this protein actively contributes to multiple aspects of mitotic progression.

Stability of kinetochore-microtubule attachment and the role of different KMN network components in Drosophila

Kinetochores bind spindle microtubules and also act as signaling centers that monitor this interaction. Defects in kinetochore assembly lead to chromosome missegregation and aneuploidy. The interaction between microtubules and chromosomes involves a conserved super-complex of proteins, known as the KNL1Mis12Ndc80 (KMN) network, composed by the KNL1 (Spc105), Mis12, and Ndc80 complexes. Previous studies indicate that all components of the network are required for kinetochore-microtubule attachment and all play relevant functions in chromosome congression, biorientation, and segregation. Here, we report a comparative study addressing the role of the different KMN components using dsRNA and in vivo fluorescence microscopy in Drosophila S2 cells allowing us to suggest that different KMN network components might perform different roles in chromosome segregation and the mitotic checkpoint signaling. Depletion of different components results in mostly lateral kinetochore-microtubule attachments that are relatively stable on depletion of Mis12 or Ndc80 but very unstable after Spc105 depletion. In vivo analysis on depletion of Mis12, Ndc80, and to some extent Spc105, shows that lateral kinetochore-microtubule interactions are still functional allowing poleward kinetochore movement. We also find that different KMN network components affect differently the localization of spindle assembly checkpoint (SAC) proteins at kinetochores. Depletion of Ndc80 and Spc105 abolishes the mitotic checkpoint, whereas depletion of Mis12 causes a delay in mitotic progression. Taken together, our results suggest that Mis12 and Ndc80 complexes help to properly orient microtubule attachment, whereas Spc105 plays a predominant role in the kinetochore-microtubule attachment as well as in the poleward movement of chromosomes, SAC response, and cell viability.

Microtubules and Alp7-Alp14 (TACC-TOG) reposition chromosomes before meiotic segregation

Tethering kinetochores at spindle poles facilitates their efficient capture and segregation by microtubules at mitotic onset in yeast. During meiotic prophase of fission yeast, however, kinetochores are detached from the poles, which facilitates meiotic recombination but may cause a risk of chromosome mis-segregation during meiosis. How cells circumvent this dilemma remains unclear. Here we show that an extensive microtubule array assembles from the poles at meiosis I onset and retrieves scattered kinetochores towards the poles to prevent chromosome drift. Moreover, the microtubule-associated protein complex Alp7-Alp14 (the fission yeast orthologues of mammalian TACC-TOG) is phosphorylated by Polo kinase, which promotes its meiosis-specific association to the outer kinetochore complex Nuf2-Ndc80 of scattered kinetochores, thereby assisting in capturing remote kinetochores. Although TOG was recently characterized as a microtubule polymerase, Dis1 (the other TOG orthologue in fission yeast), together with the Dam1 complex, plays a role in microtubule shortening to pull kinetochores polewards. Thus, microtubules and their binding proteins uniquely reconstitute chromosome configuration during meiosis.

The internal loop of fission yeast Ndc80 binds Alp7/TACC-Alp14/TOG and ensures proper chromosome attachment

The Ndc80 outer kinetochore complex plays a critical role in kinetochore-microtubule attachment, yet our understanding of the mechanism by which this complex interacts with spindle microtubules for timely and accurate chromosome segregation remains limited. Here we address this issue using an ndc80 mutant (ndc80-NH12) from fission yeast that contains a point mutation within a ubiquitous internal loop. This mutant is normal for assembly of the Ndc80 complex and bipolar spindle formation yet defective in proper end-on attachment to the spindle microtubule, with chromosome alignment defects and missegregation happening later during mitosis. We find that ndc80-NH12 exhibits impaired localization of the microtubule-associated protein complex Alp7/transforming acidic coiled coil (TACC)-Alp14/tumor-overexpressed gene (TOG) to the mitotic kinetochore. Consistently, wild-type Ndc80 binds these two proteins, whereas the Ndc80-NH12 mutant protein displays a substantial reduction of interaction. Crucially, forced targeting of Alp7-Alp14 to the outer kinetochore rescues ndc80-NH12-mutant phenotypes. The loop was previously shown to bind Dis1/TOG, by which it ensures initial chromosome capture during early mitosis. Strikingly, ndc80-NH12 is normal in Dis1 localization. Genetic results indicate that the loop recruits Dis1/TOG and Alp7/TACC-Alp14/TOG independently. Our work therefore establishes that the Ndc80 loop plays sequential roles in spindle-kinetochore attachment by connecting the Ndc80 complex to Dis1/TOG and Alp7/TACC-Alp14/TOG.

Establishment of the vertebrate kinetochores

The centromere is essential for accurate chromosome segregation during mitosis and meiosis to achieve transmission of genetic information to daughter cells. To facilitate accurate chromosome segregation, the centromere serves several specific functions, including microtubule binding, spindle-checkpoint control, and sister chromatid cohesion. The kinetochore is formed on the centromere to achieve these functions. To understand kinetochore structure and function, it is critical to identify the protein components of the kinetochore and characterize the functional properties of each component. Here, we review recent progress with regard to the molecular architecture of the kinetochore and discuss the future directions for centromere biology.

Sos7, an essential component of the conserved Schizosaccharomyces pombe Ndc80-MIND-Spc7 complex, identifies a new family of fungal kinetochore proteins

Chromosome segregation is powered by the kinetochore, a large macromolecular structure assembled on centromeric chromatin. Attachment of sister chromatids to microtubules is mediated by the highly conserved tripartite KMN (acronym for KNL-1-Mis12-Ndc80) kinetochore network. In the fission yeast Schizosaccharomyces pombe, the equivalent complex is called NMS (Ndc80-MIND-Spc7). Here, we show that not all components of the NMS complex had been identified previously. A 10th NMS component exists, the essential Sos7 protein, which is a genetic and physical interaction partner of Spc7. The analysis of sos7 kinetochore-null mutant yeast strains demonstrated that Sos7 is central to NMS function. In particular, Sos7 is required for kinetochore targeting of Spc7 as well as components of the MIND complex. sos7 mutant strains show severe chromosome missegregation phenotypes and have compromised microtubule-kinetochore interactions. Sos7 is the founding member of a functionally conserved fungal kinetochore family not present in the point centromere carrying Saccharomycotina clusters, suggesting that the new Sos7 family might be a signature motif of fungi with regional centromeres.

mDia3-EB1-APC: A connection between kinetochores and microtubule plus ends

Kinetochores must continuously associate with dynamic microtubule plus ends, as they oscillate along the mitotic spindle. The molecular basis for the kinetochore to track microtubule plus ends remains unresolved. In a recent study, we have shown an essential role of the formin mDia3 in stable kinetochore microtubule attachment and metaphase chromosome alignment. This function is attributable to EB1-binding by mDia3, for replacing endogenous mDia3 with an EB1-binding deficient mutant results in chromosome misalignment. EB1 specifically targets to attached, antipoleward kinetochores with polymerizing microtubules during chromosome oscillation. Therefore, we speculate that the mDai3-EB1-APC complex formation may relay EB1 microtubule plus end-tracking activity to the kinetochore.

Asp1, a conserved 1/3 inositol polyphosphate kinase, regulates the dimorphic switch in Schizosaccharomyces pombe

The ability to undergo dramatic morphological changes in response to extrinsic cues is conserved in fungi. We have used the model yeast Schizosaccharomyces pombe to determine which intracellular signal regulates the dimorphic switch from the single-cell yeast form to the filamentous invasive growth form. The S. pombe Asp1 protein, a member of the conserved Vip1 1/3 inositol polyphosphate kinase family, is a key regulator of the morphological switch via the cAMP protein kinase A (PKA) pathway. Lack of a functional Asp1 kinase domain abolishes invasive growth which is monopolar, while an increase in Asp1-generated inositol pyrophosphates (PP) increases the cellular response. Remarkably, the Asp1 kinase activity encoded by the N-terminal part of the protein is regulated negatively by the C-terminal domain of Asp1, which has homology to acid histidine phosphatases. Thus, the fine tuning of the cellular response to environmental cues is modulated by the same protein. As the Saccharomyces cerevisiae Asp1 ortholog is also required for the dimorphic switch in this yeast, we propose that Vip1 family members have a general role in regulating fungal dimorphism.

Regulated targeting of protein phosphatase 1 to the outer kinetochore by KNL1 opposes Aurora B kinase

Regulated interactions between kinetochores and spindle microtubules are essential to maintain genomic stability during chromosome segregation. The Aurora B kinase phosphorylates kinetochore substrates to destabilize kinetochore-microtubule interactions and eliminate incorrect attachments. These substrates must be dephosphorylated to stabilize correct attachments, but how opposing kinase and phosphatase activities are coordinated at the kinetochore is unknown. Here, we demonstrate that a conserved motif in the kinetochore protein KNL1 directly interacts with and targets protein phosphatase 1 (PP1) to the outer kinetochore. PP1 recruitment by KNL1 is required to dephosphorylate Aurora B substrates at kinetochores and stabilize microtubule attachments. PP1 levels at kinetochores are regulated and inversely proportional to local Aurora B activity. Indeed, we demonstrate that phosphorylation of KNL1 by Aurora B disrupts the KNL1-PP1 interaction. In total, our results support a positive feedback mechanism by which Aurora B activity at kinetochores not only targets substrates directly, but also prevents localization of the opposing phosphatase.

Roles for the conserved spc105p/kre28p complex in kinetochore-microtubule binding and the spindle assembly checkpoint

BACKGROUND

Kinetochores attach sister chromatids to microtubules of the mitotic spindle and orchestrate chromosome disjunction at anaphase. Although S. cerevisiae has the simplest known kinetochores, they nonetheless contain approximately 70 subunits that assemble on centromeric DNA in a hierarchical manner. Developing an accurate picture of the DNA-binding, linker and microtubule-binding layers of kinetochores, including the functions of individual proteins in these layers, is a key challenge in the field of yeast chromosome segregation. Moreover, comparison of orthologous proteins in yeast and humans promises to extend insight obtained from the study of simple fungal kinetochores to complex animal cell kinetochores.

PRINCIPAL FINDINGS

We show that S. cerevisiae Spc105p forms a heterotrimeric complex with Kre28p, the likely orthologue of the metazoan kinetochore protein Zwint-1. Through systematic analysis of interdependencies among kinetochore complexes, focused on Spc105p/Kre28p, we develop a comprehensive picture of the assembly hierarchy of budding yeast kinetochores. We find Spc105p/Kre28p to comprise the third linker complex that, along with the Ndc80 and MIND linker complexes, is responsible for bridging between centromeric heterochromatin and kinetochore MAPs and motors. Like the Ndc80 complex, Spc105p/Kre28p is also essential for kinetochore binding by components of the spindle assembly checkpoint. Moreover, these functions are conserved in human cells.

CONCLUSIONS/SIGNIFICANCE

Spc105p/Kre28p is the last of the core linker complexes to be analyzed in yeast and we show it to be required for kinetochore binding by a discrete subset of kMAPs (Bim1p, Bik1p, Slk19p) and motors (Cin8p, Kar3p), all of which are nonessential. Strikingly, dissociation of these proteins from kinetochores prevents bipolar attachment, even though the Ndc80 and DASH complexes, the two best-studied kMAPs, are still present. The failure of Spc105 deficient kinetochores to bind correctly to spindle microtubules and to recruit checkpoint proteins in yeast and human cells explains the observed severity of missegregation phenotypes.

The life and miracles of kinetochores

Kinetochores are large protein assemblies built on chromosomal loci named centromeres. The main functions of kinetochores can be grouped under four modules. The first module, in the inner kinetochore, contributes a sturdy interface with centromeric chromatin. The second module, the outer kinetochore, contributes a microtubule-binding interface. The third module, the spindle assembly checkpoint, is a feedback control mechanism that monitors the state of kinetochore-microtubule attachment to control the progression of the cell cycle. The fourth module discerns correct from improper attachments, preventing the stabilization of the latter and allowing the selective stabilization of the former. In this review, we discuss how the molecular organization of the four modules allows a dynamic integration of kinetochore-microtubule attachment with the prevention of chromosome segregation errors and cell-cycle progression.

Intrakinetochore localization and essential functional domains of Drosophila Spc105

The kinetochore is assembled during mitotic and meiotic divisions within the centromeric region of chromosomes. It is composed of more than eighty different proteins. Spc105 (also designated as Spc7, KNL-1 or Blinkin in different eukaryotes) is a comparatively large kinetochore protein, which can bind to the Mis12/MIND and Ndc80 complexes and to the spindle assembly checkpoint components Bub1 and BubR1. Our genetic characterization of Drosophila Spc105 shows that a truncated version lacking the rapidly evolving, repetitive central third still provides all essential functions. Moreover, in comparison with Cenp-C that has previously been observed to extend from the inner to the outer kinetochore region, full-length Spc105 is positioned further out and is not similarly extended along the spindle axis. Thus, our results indicate that Spc105 forms neither an extended link connecting inner Cenp-A chromatin with outer kinetochore regions nor a scaffold constraining kinetochore subcomplexes and spindle assembly checkpoint components together into a geometrically rigid supercomplex. Spc105 seems to provide a platform within the outer kinetochore allowing independent assembly of various kinetochore components.

Human Blinkin/AF15q14 is required for chromosome alignment and the mitotic checkpoint through direct interaction with Bub1 and BubR1

The spindle checkpoint controls mitotic progression. Checkpoint proteins are temporally recruited to kinetochores, but their docking site is unknown. We show that a human kinetochore oncoprotein, AF15q14/blinkin, a member of the Spc105/Spc7/KNL-1 family, directly links spindle checkpoint proteins BubR1 and Bub1 to kinetochores and is required for spindle checkpoint and chromosome alignment. Blinkin RNAi causes accelerated mitosis due to a checkpoint failure and chromosome misalignment resulting from the lack of kinetochore and microtubule attachment. Blinkin RNAi phenotypes resemble the double RNAi phenotypes of Bub1 and BubR1 in living cells. While the carboxy domain associates with the c20orf172/hMis13 and DC8/hMis14 subunits of the hMis12 complex in the inner kinetochore, association of the amino and middle domain of blinkin with the TPR domains in the amino termini of BubR1 and Bub1 is essential for BubR1 and Bub1 to execute their distinct mitotic functions. Blinkin may be the center of the network for generating kinetochore-based checkpoint signaling.

KNL1 and the CENP-H/I/K complex coordinately direct kinetochore assembly in vertebrates

Chromosome segregation during mitosis requires the assembly of a large proteinaceous structure termed the kinetochore. In Caenorhabditis elegans, KNL-1 is required to target multiple outer kinetochore proteins. Here, we demonstrate that the vertebrate KNL1 counterpart is essential for chromosome segregation and is required to localize a subset of outer kinetochore proteins. However, unlike in C. elegans, depletion of vertebrate KNL1 does not abolish kinetochore localization of the microtubule-binding Ndc80 complex. Instead, we show that KNL1 and CENP-K, a subunit of a constitutively centromere-associated complex that is missing from C. elegans, coordinately direct Ndc80 complex localization. Simultaneously reducing both hKNL1 and CENP-K function abolishes all aspects of kinetochore assembly downstream of centromeric chromatin and causes catastrophic chromosome segregation defects. These findings explain discrepancies in kinetochore assembly pathways between different organisms and reveal a surprising plasticity in the assembly mechanism of an essential cell division organelle.

The CENP-A NAC/CAD kinetochore complex controls chromosome congression and spindle bipolarity

Kinetochores are complex protein machines that link chromosomes to spindle microtubules and contain a structural core composed of two conserved protein-protein interaction networks: the well-characterized KMN (KNL1/MIND/NDC80) and the recently identified CENP-A NAC/CAD. Here we show that the CENP-A NAC/CAD subunits can be assigned to one of two different functional classes; depletion of Class I proteins (Mcm21R(CENP-O) and Fta1R(CENP-L)) causes a failure in bipolar spindle assembly. In contrast, depletion of Class II proteins (CENP-H, Chl4R(CENP-N), CENP-I and Sim4R(CENP-K)) prevents binding of Class I proteins and causes chromosome congression defects, but does not perturb spindle formation. Co-depletion of Class I and Class II proteins restores spindle bipolarity, suggesting that Class I proteins regulate or counteract the function of Class II proteins. We also demonstrate that CENP-A NAC/CAD and KMN regulate kinetochore-microtubule attachments independently, even though CENP-A NAC/CAD can modulate NDC80 levels at kinetochores. Based on our results, we propose that the cooperative action of CENP-A NAC/CAD subunits and the KMN network drives efficient chromosome segregation and bipolar spindle assembly during mitosis.

Structures and functions of yeast kinetochore complexes

The kinetochore is a key cell division organelle that enables high-fidelity transmission of genetic information by coupling chromosomes to spindle microtubules during mitosis and meiosis. Despite its cytological description more than a century ago, remarkably little information is available on kinetochore function at a molecular level. Recently, important advances elucidating the overall organization of kinetochores, as well as information about the structures and molecular mechanisms of kinetochore function, have been achieved through a detailed analysis of the kinetochores of the budding yeast Saccharomyces cerevisiae. Here we review the current understanding of kinetochore function in budding yeast and draw comparisons to recent findings in other organisms.

The conserved Spc7 protein is required for spindle integrity and links kinetochore complexes in fission yeast

Spc7, a member of the conserved Spc105/KNL-1 family of kinetochore proteins, was identified as an interaction partner of the EB1 homologue Mal3. Spc7 associates with the central centromere region of the chromosome but does not affect transcriptional silencing. Here, we show that Spc7 is required for the integrity of the spindle as well as for targeting of MIND but not of Ndc80 complex components to the kinetochore. Spindle defects in spc7 mutants were severe ranging from the inability to form a bipolar spindle in early mitosis to broken spindles in midanaphase B. spc7 mutant phenotypes were partially rescued by extra alpha-tubulin or extra Mal2. Thus, Spc7 interacts genetically with the Mal2-containing Sim4 complex.

Monitoring dynamic systems with multiparameter fluorescence imaging

A new general strategy based on the use of multiparameter fluorescence detection (MFD) to register and quantitatively analyse fluorescence images is introduced. Multiparameter fluorescence imaging (MFDi) uses pulsed excitation, time-correlated single-photon counting and a special pixel clock to simultaneously monitor the changes in the eight-dimensional fluorescence information (fundamental anisotropy, fluorescence lifetime, fluorescence intensity, time, excitation spectrum, fluorescence spectrum, fluorescence quantum yield, distance between fluorophores) in real time. The three spatial coordinates are also stored. The most statistically efficient techniques known from single-molecule spectroscopy are used to estimate fluorescence parameters of interest for all pixels, not just for the regions of interest. Their statistical significance is judged from a stack of two-dimensional histograms. In this way, specific pixels can be selected for subsequent pixel-based subensemble analysis in order to improve the statistical accuracy of the parameters estimated. MFDi avoids the need for sequential measurements, because the registered data allow one to perform many analysis techniques, such as fluorescence-intensity distribution analysis (FIDA) and fluorescence correlation spectroscopy (FCS), in an off-line mode. The limitations of FCS for counting molecules and monitoring dynamics are discussed. To demonstrate the ability of our technique, we analysed two systems: (i) interactions of the fluorescent dye Rhodamine 110 inside and outside of a glutathione sepharose bead, and (ii) microtubule dynamics in live yeast cells of Schizosaccharomyces pombe using a fusion protein of Green Fluorescent Protein (GFP) with Minichromosome Altered Loss Protein 3 (Mal3), which is involved in the dynamic cycle of polymerising and depolymerising microtubules.

Fta2, an essential fission yeast kinetochore component, interacts closely with the conserved Mal2 protein

The fission yeast multiprotein-component Sim4 complex plays a fundamental role in the assembly of a functional kinetochore. It affects centromere association of the histone H3 variant CENP-A as well as kinetochore association of the DASH complex. Here, multicopy suppressor analysis of a mutant version of the Sim4 complex component Mal2 identified the essential Fta2 kinetochore protein, which is required for bipolar chromosome attachment. Kinetochore localization of Mal2 and Fta2 depends on each other, and overexpression of one protein can rescue the phenotype of the mutant version of the other protein. fta2 mal2 double mutants were inviable, implying that the two proteins have an overlapping function. This close interaction with Fta2 is not shared by other Sim4 complex components, indicating the existence of functional subgroups within this complex. The Sim4 complex seems to be assembled in a hierarchical way, because Fta2 is localized correctly in a sim4 mutant. However, Fta2 kinetochore localization is reduced in a spc7 mutant. Spc7, a suppressor of the EB1 family member Mal3, is part of the conserved Ndc80-MIND-Spc7 kinetochore complex.

The V260I mutation in fission yeast alpha-tubulin Atb2 affects microtubule dynamics and EB1-Mal3 localization and activates the Bub1 branch of the spindle checkpoint

We have identified a novel temperature-sensitive mutant of fission yeast alpha-tubulin Atb2 (atb2-983) that contains a single amino acid substitution (V260I). Atb2-983 is incorporated into the microtubules, and their overall structures are not altered noticeably, but microtubule dynamics is compromised during interphase. atb2-983 displays a high rate of chromosome missegregation and is synthetically lethal with deletions in a subset of spindle checkpoint genes including bub1, bub3, and mph1, but not with mad1, mad2, and mad3. During early mitosis in this mutant, Bub1, but not Mad2, remains for a prolonged period in the kinetochores that are situated in proximity to one of the two SPBs (spindle pole bodies). High dosage mal3(+), encoding EB1 homologue, rescues atb2-983, suggesting that Mal3 function is compromised. Consistently, Mal3 localization and binding between Mal3 and Atb2-983 are impaired significantly, and a mal3 single mutant, such as atb2-983, displays prolonged Bub1 kinetochore localization. Furthermore in atb2-983 back-and-forth centromere oscillation during prometaphase is abolished. Intriguingly, this oscillation still occurs in the mal3 mutant, indicating that there is another defect independent of Mal3. These results show that microtubule dynamics is important for coordinated execution of mitotic events, in which Mal3 plays a vital role.

Mal3, the fission yeast EB1 homologue, cooperates with Bub1 spindle checkpoint to prevent monopolar attachment

Bipolar microtubule attachment is central to genome stability. Here, we investigate the mitotic role of the fission yeast EB1 homologue Mal3. Mal3 shows dynamic inward movement along the spindle, initial emergence at the spindle pole body (SPB) and translocation towards the equatorial plane, followed by sudden disappearance. Deletion of Mal3 results in early mitotic delay, which is dependent on the Bub1, but not the Mad2, spindle checkpoint. Consistently, Bub1, but not Mad2, shows prolonged kinetochore localization. Double mutants between mal3 and a subset of checkpoint mutants, including bub1, bub3, mad3 and mph1, but not mad1 or mad2, show massive chromosome mis-segregation defects. In mal3bub1 mutants, both sister centromeres tend to remain in close proximity to one of the separating SPBs. Further analysis indicates that mis-segregated centromeres are exclusively associated with the mother SPB. Mal3, therefore, has a role in preventing monopolar attachment in cooperation with the Bub1/Bub3/Mad3/Mph1-dependent checkpoint.

The kinetochore and cancer: what's the connection?

The molecular mechanisms ensuring accurate chromosome segregation during meiosis and mitosis are critical to the conservation of euploidy (normal chromosome number) in eukaryotic cells. A dysfunctional kinetochore represents one possible source for chromosome instability (CIN) and the generation of aneuploidy. The kinetochore is a large complex of proteins and associated centromeric DNA that is responsible for mediating the segregation of sister chromatids to daughter cells via its interactions with the mitotic spindle. Continued identification of conserved kinetochore components in model systems such as yeast has provided a rich resource of candidate genes that may be mutated or misregulated in human cancers. Systematic mutational testing and transcriptional profiling of CIN candidate kinetochore genes should shed light on the kinetochore's role in tumorigenesis, and on the general role CIN plays in cancer development.

Molecular analysis of kinetochore architecture in fission yeast

Kinetochore composition and structure are critical for understanding how kinetochores of different types perform similar functions in chromosome segregation. We used affinity purification to investigate the kinetochore composition and assembly in Schizosaccharomyces pombe. We identified a conserved DASH complex that functions to ensure precise chromosome segregation. Unlike DASH in budding yeast that is localized onto kinetochores throughout the cell cycle, SpDASH is localized onto kinetochores only in mitosis. We also identified two independent groups of kinetochore components, one of which, the Sim4 complex, contains several novel Fta proteins in addition to known kinetochore components. DASH is likely to be associated with the Sim4 complex via Dad1 protein. The other group, Ndc80-MIND-Spc7 complex, contains the conserved Ndc80 and MIND complexes and Spc7 protein. We propose that fission yeast kinetochore is comprised of at least two major structural motifs that are biochemically separable. Our results suggest a high degree of conservation between the kinetochores of budding yeast and fission yeast even though many individual protein subunits do not have a high degree of sequence similarity.

The DASH complex and Klp5/Klp6 kinesin coordinate bipolar chromosome attachment in fission yeast

We identified a truncated allele of dam1 as a multicopy suppressor of the sensitivity of cdc13-117 (cyclin B) and mal3-1 (EB-1) cells to thiabendazole, a microtubule poison. We find that Dam1 binds to the plus end of spindle microtubules and kinetochores as cells enter mitosis and this is dependent on other components of the fission yeast DASH complex, including Ask1, Duo1, Spc34 and Dad1. By contrast, Dad1 remains bound to kinetochores throughout the cell cycle and its association is dependent on the Mis6 and Mal2, but not Mis12, Nuf2 or Cnp1, kinetochore proteins. In cells lacking Dam1, or other components of the DASH complex, anaphase is delayed due to activation of the spindle assembly checkpoint and lagging sister chromatids are frequently observed and occasionally sister chromatid pairs segregate to the same spindle pole. We find that the mitotic centromere-associated Klp5/Klp6 kinesin complex is essential in cells lacking components of the DASH complex. Cells lacking both Dam1 and Klp5 undergo a first cell cycle arrest in mitosis due to a failure to establish bipolar chromosome attachment.